1. Technical Field
The invention relates generally to MOS circuits. More specifically, the invention relates to improving drain-source leakage in deep submicron MOS transistors.
2. Discussion of Related Art
The advantages of using dynamically adjustable threshold voltages in metal oxide semiconductor (MOS) transistors, with regard to enhancing drive-current or reducing leakage current, is known. Two general types of approaches are presently known in the art. One approach attempts to create a dynamic threshold voltage by using simple active or passive elements, such as diodes, resistors, and/or capacitors to cause such change of the threshold voltage. Another class of prior art solutions uses additional MOS transistors to control the threshold voltage in a manner that reduces leakage current.
U.S. Pat. Nos. 7,224,205, 6,521,948, and 5,821,769, teach inventions that make use of a diode for the purpose of controlling leakage in MOS transistors. These solutions, as well as others known in the art, are not fully compliant in one way or another with standard CMOS process flows. This is because a general MOS process flow does not allow the creation of diodes in the general active area, although diodes some are allowed at the pad region of an integrated circuit (IC). The design rules do not allow diodes because this is thought to prevent errors in the design that may cause the faulty operation of the IC, or even permanent damage to the IC. To enable such solutions as may control leakage, the design rules must be relaxed and, in some prior art solutions, changes to the process flow are also required.
U.S. Pat. Nos. 6,952,113, 6,744,301, 6,441,647, 6,304,110, 6,291,857, 6,118,328, 5,994,177, and 5,644,266 teach examples of solutions that use additional circuitry comprising MOS transistors to achieve reduced leakage of the MOS circuit. The MOS transistors that are controlled by these circuits connect the fourth terminal of the MOS transistor, i.e. the body, the other three terminals being the gate, source and drain, to an appropriate voltage to control the threshold voltage in a desired manner. Various solutions use a different number of transistors to control the leakage of the transistors and generally have a larger impact on a cell size than the first approach discussed above. These solutions have the advantage of not deviating from the standard process. They suffer, however, from various shortcomings, including an unpredictable body potential due to leakage currents and other factors, and a requirement for a direct current (DC) input draw.
There is a therefore a need in the art for a circuit which can reduce the drain-source leakage of MOS transistors and further overcome the deficiencies of prior art solutions.